This invention is related to an improved method for the separation of a feed stream into a relatively low boiling distillate and a relatively high boiling bottoms stream. More specifically, this invention is directed at an improved, energy efficient method for separating a petroleum fraction into a distillate having a relatively low boiling point and a bottoms having a relatively high boiling point.
In the distillation of a liquid to separate the liquid into a distillate and a bottoms, steam frequently is added to impart heat and to aid in the separation. In the petroleum industry, steam frequently is added to the distillation column during the separation of feeds, such as vacuum residuum to improve the separation of the lighter components from the heavier components.
Often, after distillation equipment has been designed and installed, the desired distillation product requirements change and/or the feed composition changes. For example, in the petroleum industry the feeds entering the distillation zone frequently have a higher boiling point than was contemplated when the equipment was designed. Often it also is desired to strip out high boiling fractions from the bottoms product. In many instances, changes in the tower operating conditions may produce the desired products. In some cases, however, changes in the column operating conditions will not produce the desired distillate and bottoms at the desired operating rates, or the changes required would make operation of the column uneconomical. In other instances, utility limitations, i.e. cooling water and/or steam supply limitations may preclude significant changes in the distillation column operating conditions. Accordingly, in many instances the present distillation columns must be completely replaced or extensively modified, such as by replacing the column internals or by adding additional sections onto the column. Replacement or extensive modification of a distillation zone may be extremely costly. In addition to the actual cost for the replacement and/or modification of the column, this work will require the shutdown of the distillation zone for an extended period of time. In addition, distillation column modification and/or replacement frequently will necessitate replacement of substantial amounts of piping, instrumentation and related equipment.
U.S. Pat. No. 2,461,694 is directed at a process for the continuous distillation of fatty material from oils. This patent describes a process requiring two extraction zones. Feed enters near the top of the first zone and is flashed into a vapor, which is easily removed, and into a liquid which is steam stripped as it passes through the extraction zone. The bottoms from the first extractor are then passed to a second extractor where it is again steam stripped. Vacuum is applied to the second extractor by a steam jet. Vapor from the second column and the uncondensed steam from the steam jet are directed into the base of the first extractor. Since this process does not provide both rectification and stripping zones in each unit, and since this process does not reflux any overheads, product separation is undesirably low.
U.S. Pat. Nos. 2,615,833 and 3,421,567 disclose the use of steam ejectors to remove a vapor side stream from one location in a column and discharge the steam and vapor into a second location in the column either above or below the first location. These methods would not be beneficial, however in applications where a discrete new product is required without increasing the existing utility usage (e.g. steam consumption).
U.S. Pat. No. 4,261,814 discloses the use of a steam jet ejector to recirculate a vapor stream from the top of a vacuum pipestill to the bottom to permit either a deeper cut in the vacuum residuum or the same yields but using less steam. This patent does not disclose a method for producing a deeper cut, where the required fractionation products could not be obtained using only an existing column.
Accordingly, it is desirable to provide a process which will permit a significant improvement in distillation zone performance without requiring extensive modification of an existing column or replacement with a new distillation column.
It is also desirable to provide improved distillation zone performance without shutting down the existing distillation zone for an extended period of time.
It is further desirable to provide a process which will result in improved distillation zone performance with little or no increase in utility consumption.
The subject invention is directed at improved distillation zone performance in which a second distillation column communicates with the first distillation column. Bottoms from the first column are directed into the second column maintained under reduced pressure by an ejector means, such as a steam jet ejector. The overheads from the second column and the motive fluid from the ejector means are passed into the rectification zone of the first column.